1. Technical Field
The present invention relates to a vehicle airbag system and, particularly, relates to an igniter for igniting gas generating propellant grains in an inflator of the airbag system.
2. Description of the Prior Art
Igniters for igniting gas generating propellant grains in inflators for vehicle airbag systems are known. One such known igniter includes a metal housing and an ignitable material contained within the housing. During combustion of the ignitable material, hot gases, flame and pressure are generated within the housing. When the pressure within the housing increases to approximately 1500 psi, the housing ruptures and the hot gases and flame escape under pressure.
When the igniter is used in an inflator having a plurality of nitrogen gas generating propellant grains with an ignition enhancing coating, such as the propellant grains disclosed in U.S. Pat. No. 4,698,107, the igniter is located coaxially of and within axially extending openings in the end propellant grains. The ignitable material in the igniter is typically ignited at one axial end of the igniter housing and burns toward the other axial end of the igniter housing which is weakened by scribe lines. The weakened areas rupture and the hot gases and flame flow axially from the igniter and strike an adjacent end surface of another propellant grain.
When the propellant grain is suddenly impacted by the hot gases and flame, the propellant grain may shatter or the ignition enhancing coating may detonate and shatter one or more of the propellant grains. A shattered propellant grain has a greater surface area than an undamaged propellant grain. The greater surface area of a shattered propellant grain accelerates combustion of the propellant grains and causes a larger volume of gas to be generated in a shorter period of time. Furthermore, the propellant grains often shatter in a manner that is not repeatable from grain to grain. Thus, shattered propellant grains may detrimentally affect the performance of the inflator. The propensity of the igniter to shatter the propellant grain because of suddenly exposing the propellant grain and/or the coating to the pressure is referred to as "brisance".
U.S. Pat. No. 2,934,014 and U.S. Pat. No. 3,011,441 recognize the problem of brisance when pressure from an igniter shatters gas generating propellant grains in a rocket motor. The igniters disclosed in U.S. Patents No. 2,934,014 and U.S. Pat. No. 3,011,441 address the brisance problem by igniting the ignitable material in an igniter housing at one axial end. The ignitable material then burns toward a closed end of the igniter housing. A pressure wave front caused by combustion of the ignitable material reflects off the closed end of the igniter housing. The reflected pressure wave then exits the igniter through a ruptured open end of the igniter housing adjacent a location at which ignition of the ignitable material started. Reflecting the pressure wave dissipates some of the energy generated by combustion of the ignitable material. The impact of the pressure wave against the propellant grain is, thus, reduced to a level which can be tolerated by the propellant grain to minimize shattering.